Re: [PATCH] f2fs: avoid race between zero_range and background GC

[Jaegeuk Kim]

On 07/29, Chao Yu wrote:
> On 2018/7/29 10:02, Jaegeuk Kim wrote:
> > On 07/27, Chao Yu wrote:
> >> On 2018/7/27 18:29, Jaegeuk Kim wrote:
> >>> On 07/26, Chao Yu wrote:
> >>>> Thread A				Background GC
> >>>> - f2fs_zero_range
> >>>>  - truncate_pagecache_range
> >>>> 					- gc_data_segment
> >>>> 					 - get_read_data_page
> >>>> 					  - move_data_page
> >>>> 					   - set_page_dirty
> >>>> 					   - set_cold_data
> >>>>  - f2fs_do_zero_range
> >>>>   - dn->data_blkaddr = NEW_ADDR;
> >>>>   - f2fs_set_data_blkaddr
> >>>>
> >>>> Actually, we don't need to set dirty & checked flag on the page, since
> >>>> all valid data in the page should be zeroed by zero_range().
> >>>
> >>> But, it doesn't matter too much, right?
> >>
> >> No, if the dirtied page is writebacked after f2fs_do_zero_range(), result of
> >> zero_range() should be wrong, as zeroed page contains valid user data.
> > 
> > How about truncating page caches after block address change or doing it twice
> > before and after?
> 
> Thread A				Background GC
> - f2fs_zero_range
>  - truncate_pagecache_range
> 					- gc_data_segment
> 					 - get_read_data_page
> 					  - move_data_page
> 					   - set_page_dirty
> 					   - set_cold_data
>  - f2fs_do_zero_range
>   - dn->data_blkaddr = NEW_ADDR;
>   - f2fs_set_data_blkaddr
> 					bdi-flusher
> 					- __write_data_page
> 					 - f2fs_update_data_blkaddr
> 					 : data_blkaddr has been updated here.
>  - truncate_pagecache_range
>  : data & dnode has been writebacked before page cache truncation?
> 
> How about this case?

So, truncating pages under dnode lock can address it?

> 
> Thanks,
> 
> > 
> >>
> >>>
> >>>> Use i_gc_rwsem[WRITE] to avoid such race condition.
> >>>
> >>> Hope to avoid abusing i_gc_rwsem[] tho.
> >>
> >> Agreed, let's try avoiding until we have to use it.
> >>
> >> Thanks,
> >>
> >>>
> >>>>
> >>>> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> >>>> ---
> >>>>  fs/f2fs/file.c | 2 ++
> >>>>  1 file changed, 2 insertions(+)
> >>>>
> >>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> >>>> index 267ec3794e1e..7bd2412a8c37 100644
> >>>> --- a/fs/f2fs/file.c
> >>>> +++ b/fs/f2fs/file.c
> >>>> @@ -1309,6 +1309,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
> >>>>  	if (ret)
> >>>>  		return ret;
> >>>>  
> >>>> +	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> >>>>  	down_write(&F2FS_I(inode)->i_mmap_sem);
> >>>>  	ret = filemap_write_and_wait_range(mapping, offset, offset + len - 1);
> >>>>  	if (ret)
> >>>> @@ -1389,6 +1390,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
> >>>>  	}
> >>>>  out_sem:
> >>>>  	up_write(&F2FS_I(inode)->i_mmap_sem);
> >>>> +	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
> >>>>  
> >>>>  	return ret;
> >>>>  }
> >>>> -- 
> >>>> 2.18.0.rc1